1. Field of the Invention
The present invention relates generally to drive systems, and more particularly to powered caster wheel modules used to create drive systems such as for a mobile robot platform, automated guided vehicle (AGV), forklift, or omnidirectional powered roller conveyor.
2. Discussion of the Prior Art
Mobile robots have been developed in a myriad array of configurations. In general, a mobile robot will typically have a base or platform for supporting drive systems, controllers, sensors, manipulators, and whatever equipment is needed to allow the robot to perform its desired tasks. A mobile robot base can be driven by wheels, tracks, xe2x80x9clegsxe2x80x9d or a variety of other means.
It is useful for a mobile robot to be very maneuverable. The more agile the robot, the better it can deal with changing goals, obstacles, environments, and tasks. This is even more important when a manipulator is integrated with a mobile robot.
The mobility of a drive system increases with the ability to control an increasing number of independent degrees of freedom. For a vehicle that moves along the ground, there are three degrees of freedom available, most commonly described as two translations and one rotation. The ability to independently control all of the degrees of freedom available in the environment means that the system is omnidirectional. The ability to independently control the acceleration of all of the degrees of freedom available in the environment means that the system is holonomic.
To help achieve greater mobility, one or more caster mounted wheels can be utilized that pivot about a vertical axis as well as rotate about a horizontal axis. This arrangement makes the drive system omnidirectional. Preferably, the vertical steering axis does not intersect the horizontal drive axis. This offset arrangement allows the caster wheels to drive the robot and accelerate it in any direction, making it holonomic. In other words, the drive system can always create planer omnidirectional accelerations, velocities, and displacements of the robot, rather than requiring the robot wheels in some orientations to skid or to drive forward before turning to the side.
A mobile robot base has been previously developed which uses four caster wheels with intersecting horizontal and vertical axes for driving and steering the base. (Although the axes intersect, the xe2x80x9ccontact patchxe2x80x9d of each wheel is offset from the vertical steering axis, making the system non-holonomic.) The driving axes of all of the wheels are linked together by a drive belt, and are driven by a single motor. Similarly, all of the steering axes of the wheels are linked together by a drive linkage, and are actuated by a second motor. However, this type of system, known as xe2x80x9csynchro-drive,xe2x80x9d has several drawbacks. Because the wheels must all drive in the same direction and at same speed at any given moment, certain complex maneuvering cannot be performed. The motion-transmitting belts and linkages also add complexity and backlash to the drive train.
Prior art mobile robot caster wheels are typically driven by a bevel gear mounted on one side the wheel, outwardly facing and concentric with the axis of rotation. Both the bevel gear and the wheel often are standard xe2x80x9coff the shelfxe2x80x9d components. Mounting holes are provided through a flange on the bevel gear for receiving fasteners to secure the gear to the wheel. The bevel gear is aligned with the center of the wheel before being secured. However, inaccuracies in aligning the gear and accumulation of tolerances between the gear and the wheel bearings prevent the gear from being located in a truly concentric fashion, and from being precisely perpendicular to the rotation axis. Inaccuracies in the manufacture of the wheel and over, under, or uneven tightening of the fasteners can prevent the bevel gear from being precisely located laterally with respect to the mating pinion. Such misalignments of the bevel gear cause the gear to turn inconsistently and wear prematurely. Also, lack of concentricity precision between the wheel tread or outer wheel circumference and the axis of rotation causes the mobile robot base to run unevenly. These problems can prevent the robot from accurately maintaining its desired trajectory.
What is needed and is not provided by the prior art is an omnidirectional or holonomic drive system that exhibits a high degree of mobility and accuracy, yet is simple, compact and reliable.
Broadly stated, a drive system constructed according to the present invention can provide a robot, vehicle, or other device with a high degree of mobility and accuracy, yet is simple, compact and reliable.
In accordance with one aspect of the present invention, a drive system is provided with multiple caster wheels, each wheel having its own separate motor for driving the wheel and its own separate motor for steering. This allows each wheel to be driven and steered independently. The motion of multiple wheels can be coordinated for increased mobility. Complex linkages interconnecting the wheels are also eliminated.
In accordance with another aspect of the invention, each wheel is mounted in a separate wheel module, which includes both the driving and steering motors associated with the wheel. This modular arrangement of powered caster wheels allows a drive system for a mobile robot base or other device to be designed and built much easier than before. Since there are no mechanical, motion-transmitting linkages between modules, each can be built and tested independently. The easily removable modules can be extracted for maintenance. Using a common module in several places in a drive system reduces the cost of the system because of the increased number of each part. Common modules are easily replaced when damaged. Modules can be fastened to different sizes and configurations of robot bases to produce different vehicles without redesigning the drive system. The design is easily scaled to produce a powered caster wheel module of any size.
In accordance with still another aspect of the invention, the drive and steering motors are arranged in compact orientation such that their armatures are both vertically aligned and their outer housings are close together. This arrangement provides an efficient use of space and a compact footprint. Because the wheel module takes up less space on the mobile robot base or device, space is made available for more wheel modules or other components.
In accordance with yet another aspect of the invention, each wheel module is individually mounted on a resilient suspension to give a robot base a suspension. This ensures that all of the wheels maintain contact with the ground for precise motion and position tracking of the robot, and allows for smooth driving of the vehicle on uneven terrain.
In accordance with yet another aspect of the invention, a bevel gear is provided on one side of each wheel for driving the wheel, and bearings for rotatably mounting the wheel are located within a precision bore through the wheel. A rigid hub and resilient tire tread are cast in place around the bevel gear to form a wheel having a bevel gear and outer tire circumference that are highly concentric with the axis of rotation of the wheel. This arrangement provides for smooth rotation of each wheel and precise alignment between each bevel gear and its associated drive pinion.